Here are the countries with the cleanest cities

Air pollution is a big problem — it's responsible for an estimated 3.3 million deaths every year.

A tiny pollutant called PM2.5, or particulate matter smaller than 2.5 microns across, is considered the most harmful pollutant. It's found in soot, smoke, and dust. When it lodges in the lungs, it can cause long-term health problems like asthma and chronic lung disease.

While we typically tend to focus on the places that are the least healthy, here's a list of the most healthy.

8. Sweden and Brunei: 4 cities each

Umeå, SwedenFlickr/Blondinrikard Fröberg

Of the top 10 countries with the most clean cities, Sweden and Brunei both have a solid four cities with seriously clean air. Notably, Sweden's two most populated cities, Stockholm and Gothenberg both had PM2.5 levels deemed healthy by WHO, and is actively working to keep its air clean. Brunei, a country surrounded by Malaysia has done a good job of keeping emissions low and maintaining forests, even with rapid industrialization.

Sweden's cleanest city - Umeå, 5 µg/m3

Brunei's cleanest city - Brunei-Muara District 5 µg/m3

7. Portugal and Finland: 5 cities each

Oulu, FinlandFlickr/Anna B

The next two countries also tied for the most cities with clean air, at 5 apiece. Portugal's second biggest city is also its cleanest, in part because it isn't as industrial as other parts of the country and serves as a trade port. Like Sweden, Finland is also a partner of the Climate and Clean Air Coalition and cutting down on carbon emissions.

Portugal's cleanest city - Porto, 7 µg/m3

Finland's cleanest cities - Oulu and Jyväskylä, 6 µg/m3

6. Spain: 11 cities

Las Palmas, a Spanish city off the coast of Morocco has an annual mean of 6 µg/m3 PM 2.5.Flickr/Ana Rey

Spain's cleanest cities, Arrecife and Las Palmas both are located off the coast of Morocco in the Canary Islands. The islands have clean air because of little industrialization, as tourism is one of their main economic drivers.

Spain's cleanest cities -Arrecife and Las Palmas, 6 µg/m3

5. New Zealand: 14 cities

Lower Hutt, New ZealandFlickr/Rosino

New Zealand has given a lot of attention to PM10, another harmful pollutant like PM2.5 (though a bit bigger in diameter), which has helped it rack up 14 cities with relatively clean air. Its two biggest cites, Auckland and Wellington, both have low annual PM2.5 means.

New Zealand's cleanest cities - Lower Hutt, Upper Hutt and Wainuiomata, which are near one another in the southwest corner of New Zealand's northern island - 5µg/m3

4. Australia: 15 cities

Sydney, AustraliaShutterstock

The two biggest cities in Australia also have the lowest air pollution. In fact, the five most populated cities in the country all have annual PM2.5 means that are considered healthy by WHO, in part, because the country mandates that there can't be more than an annual PM 2.5 mean of 8µg/m3.

3. Japan: 18 cities

Chikusa-ku, Nagoya, JapanFlickr/Ray Larabie

Even though the world's most populated metropolitan didn't make the list, Japan's most clean cities, the 18 that did make the cut are still an impressive bunch. Japan has been a long-time leader in clean air intiatives dating back to the Kyoto Protocol of 1992, which gave the commitment of countries to cut down on greenhouse gas emissions.

2. Canada: 115 cities

Toronto, Canada, which has an annual mean of 8 µg/m3 PM 2.5.Wikimedia Commons

Of the countries with the least amount of air pollution, Canada, which comes in second in number, has the cleanest of the clean. Half of the provinces and territories capitals had an annual PM2.5 mean that's considered healthy, including Toronto, Ottawa, and Vancouver. Canada has been attentive to keeping its air clean, even having a Clean Air Day to draw attention to harmful pollutant.

And even though Canada comes second on this list, it does have the city with the least amount of air pollution: Powell River, British Columbia. Of the more than 1,600 cities monitored by the WHO, Powell River was the only one with a seriously low annual PM2.5 mean of 2 µg/m3.

1. United States: 274 cities

Prescott, ArizonaFlickr/Ken Lund

The United States tops the WHO's list as the country with the most cities with clean air with a whopping 274, though to be fair, 69 of those were cities with 10µg/m3 — right at the cutoff point. With the help of the EPA attention to PM2.5 and clean air initiatives, this gives some hope that there are places in the states where you can get a big gulp of clean air.